Measurement using an enhanced biomolecular identification function such as coupling of antigen-antibody reaction or DNA fragment (DNA probe) and DNA is an important technique for measurement in the field of laboratory testing and biochemistry and measurement of environmental pollution substances.
Examples include micro total analysis systems (TASs), micro combinatorial chemistry, chemical IC, chemical sensors, bio sensors, micro analysis, electrochemistry analysis, QCM measurement, SPR measurement, ATR measurement, and so on, but in such measurement fields, a small amount of a sample liquid is mostly used.
As a method of measuring a chemical substance in a sample liquid, an optical measurement method in which a molecule selective substance is fixed in advance, the sample liquid is caused to flow through the molecule selective substance, and coupled molecules are selectively detected, is known. As a type of the optical measurement method, there is a method using a total reflection optical system. In this method, coupling near a surface can be measured directly and with high sensitivity using an excited evanescent wave as a probe light.
In the total reflection optical system, a surface plasmon resonance (SPR) measurement method using the fact that an evanescent wave excited by total reflection resonates with surface plasmon of a surface of a thin metallic film formed on a substrate surface and absorbed is particularly widely used.
When a sample liquid is measured using the surface plasmon resonance measurement method, a groove is formed on a substrate to provide a micro flow channel, probe molecules are fixed on a metal film disposed in the flow channel, and the sample liquid is caused to pass through the flow channel. Based on interaction between the probe molecules and target substance in the sample liquid at this time, measurement is performed to determine whether the target substance is contained in the sample liquid (see, for example, Patent Document 1 or 2).
Also, as a method of analyzing a small amount of a sample liquid, an analysis method using paper chromatography is known. For example, as simple inexpensive means for biological substance measurement, an enhanced immunochromatography method, an immuneconcentration method, and so on, have been proposed (see, for example, Patent Document 3 or 4). Even in these methods, a sample liquid is required to pass through a micro flow channel on a substrate.
In the above-described measurements and analyses, a small amount of a sample liquid is transferred to a detector so that a higher sensitivity and efficiency measurement can be performed without causing a reduction in the concentration of the sample liquid. As techniques for realizing transfer of a small amount of a solution, there are a method of forming a flow channel having a width of hundreds of μm on a substrate and transferring a solution under external pressure, a method of transferring a solution using electrostatic force, an electro-wetting method, a method of transferring a solution using a volume change or air bubble generation caused by heating, a method using electro-osmotic flow, and so on.
However, in order to transfer a small amount of a sample liquid using such methods, a flow channel needs to be formed on a substrate and other components need to be provided on the same substrate. Accordingly, it is difficult to fabricate such a device. Also, for example, where the sample liquid is transferred under external pressure, separate parts such as a pump or a tube are necessary in addition to the substrate constituting the flow channel. As a result, a transfer path, such as a tube, makes dead volume of the sample liquid, which limits use to a small amount of the sample liquid.
Accordingly, a method of forming a region of a flow channel or a pump that transfers a sample liquid using a capillary force between opposing surfaces of two substrates using a micro machining technique has been proposed (see, for example, Patent Document 5 and Non-Patent Document 1 or 2). A measuring chip (flow cell) fabricated using this technique includes an inlet through which a sample liquid is introduced, and a pump which suctions the sample liquid using capillary force. When the sample liquid is introduced to the inlet, the sample liquid flows sequentially from the inlet to a measurement flow channel and the pump, and when the sample liquid reaches the capillary pump, the sample liquid is suctioned due to a capillary force occurring in the pump. Accordingly, the sample liquid introduced in the inlet flows through the measurement flow channel due to a suction capillary force of the pump.
However, in the flow cell as described above, since a change due to binding reaction of probe molecules fixed to the metal film and target substance and a change due to a foreign substance precipitated and sedimented on the probe molecules are not discriminated, it is necessary to continuously flow the sample fluid in the measurement flow channel by suppressing precipitation of the foreign substance and improving measurement precision.
Also, it is necessary to flow the sample fluid in the measurement flow channel by a time required to detect the target substance and necessary to flow a greater amount of the sample fluid when the concentration of the target substance is decreased.
According to the requirements, the capacity of the pump can be increased for a greater amount of flowing sample fluid. However, in this case, it is effective to form a measuring chip that is large in a height direction to obtain a structure having a high aspect ratio, thereby reserving a volume of the pump in a height direction inside the measuring chip.
Meanwhile, in order to obtain a small measuring chip, it is necessary to improve efficiency of space use of the flow channel or the pump in the measuring chip. Accordingly, it is desirable to form components, such as a flow channel or a pump, in all regions inside the measuring chip in the height direction.
However, in techniques of Patent Document 5 and Non-Patent Document 2, since a flow channel formed on a substrate has an additional function to serve as a capillary pump, the capacity of the capillary pump is necessarily limited to a height range of the flow channel.
That is, a height of the capillary pump ranges from about 10 μm to about 100 μm as described in Patent Document 5 and is 30 μm as described in Non-Patent Document 2. Such a limit is caused by providing an additional function to the capillary pump, which has a unique function of flowing the sample liquid.
Accordingly, in order to sufficiently reserve the capacity of the pump, it is necessary to extend the measuring chip in a plane direction, which makes it difficult to fabricate the measuring chip with a small size and at a low cost.
Meanwhile, since lithography or etching is performed on a substrate formed of a material or a substrate itself having a groove as a flow channel is fabricated by injection molding to fabricate the measuring chip, it is difficult to fabricate a structure of a high aspect ratio. Accordingly, since the capacity of the pump is limited, the amount of the sample liquid flowing through the measurement flow channel is also limited and the sample liquid cannot be continuously flowed for sufficient measurement.
Also, in the fabricating method, because it is difficult to process the inside of the substrate, locations in the substrate at which a flow channel or a pump can be fabricated are limited, making it impossible to fabricate a structure with a high efficient use of space.
Patent Document 1: Japanese Patent Application, First Publication No. 2001- 194298
Patent Document 2: Japanese Patent Application, First Publication No. 2002- 214131
Patent Document 3: Japanese Patent Application, Second Publication No. H7-036017
Patent Document 4: Japanese Patent Application, First Publication No. 2000- 329766
Patent Document 5: Patent Application Publication No. 2005-532151
Non-Patent Document 1: Amal. Chem. 2005, 77, 7901-7907.
Non-Patent Document 2: M. Zimmermann, et al., “Capillary pumps for autonomous capillary systems,” The Royal Society of Chemistry, Lob on a Chip, Vol. 7, pp. 119-125, 2007.